Version 2: 11-month SARS-CoV-2 immunogenicity decay, and associated factors, among mRNA vaccinees: Implications for Booster Vaccination

Background We examined the 11-month longitudinal antibody decay among 2-dose mRNA vaccinees, and identified factors associated with faster decay.

Methods The study included samples from the CORSIP longitudinal observational study of paramedics in Canada. Participants were included if they had received two mRNA vaccines without prior SARS-CoV-2 infection and provided two blood samples post-vaccination. The outcomes of interest were quantitative SARS-CoV-2 antibody concentrations. We employed spaghetti and scatter plots (with kernel-weighted local polynomial smoothing curve) to describe the trend of the antibody decay over 11-months post vaccine and fit a mixed effect exponential decay model to examine the loss of immunogenicity and factors associated with antibody waning over time.

Results This analysis included 652 blood samples from 326 adult paramedics. Total anti-spike antibody levels peaked on the 21st day (antibody level 9,042U/mL) after the second mRNA vaccine dose. Total anti-spike antibody levels declined thereafter, with a half-life of 94 [95% CI: 70, 143] days, with levels plateauing at 295 days (antibody level 1021 U/mL). Older age, vaccine dosing interval <35 days, and the BNT162b2 vaccine (compared to mRNA-1273 vaccine) were associated with faster antibody decay.

Conclusion Antibody levels declined after the initial mRNA series with a half-life of 94 days, plateauing at 295 days. These findings may inform the timing of booster vaccine doses and identifying individuals with faster antibody decay.